Moderna Stock Nearly Triples in a Day -- Cancer Breakthrough, Flu Shot Win, or Just a Short Squeeze?
On August 19, 2026, Moderna shares skyrocketed as much as 177% intraday -- the biggest single-day move in company history -- after Moderna and Merck released 5-year Phase 3 data showing their personalized mRNA cancer vaccine, combined with Keytruda, sustained a meaningful reduction in melanoma recurrence. The rally came just two weeks after the FDA approved mFlusiva, Moderna's first mRNA-based flu vaccine for adults 50+, the first mRNA flu shot ever authorized in the US. Because Moderna was heavily shorted going into the announcement, the surge reportedly inflicted roughly $5.5 billion in one-day losses on short sellers, fueling debate over how much of the move is short covering versus a genuine re-rating of the company's pipeline. Reaction has split Wall Street, with some newly bullish on Moderna's expanding mRNA platform, while others warn the stock's valuation has run well ahead of its shrinking COVID/RSV vaccine revenue base.
